A W-CDMA system is one of wireless communication interfaces provided by IMT-2000 (International Mobile Telecommunications-2000). The W-CDMA system which is the mainstream of a wireless communication system has a maximum transmission rate of 384 (kbps) to enable an access to multimedia such as a sound, a moving image and data. As a communication system based on the communication technique of this W-CDMA system, a study and development of a HSDPA (High Speed Downlink Packet Access) system are being advanced. This HSDPA system performs high speed downlink packet transmission downstream, and realizes a maximum transmission rate of 14.4 (Mbps).
Concerning power control in such a wireless communication system, Japanese Laid-open Patent Publication No. 2005-303836 discloses that an A-DPCH transmission power setting section, a target error rate setting section and an HS-SCCH transmission power setting section are provided, and the A-DPCH transmission power setting section sets transmission power to each mobile station device (MS) to which A-DPCH is assigned, the target error rate setting section individually sets, based on the environment of propagation with each MS, a target reception error rate Target-BLER of HS-SCCH when control information is transmitted to each MS, and the HS-SCCH transmission power setting section sets, based on the set transmission power and target reception error rate Target-BLER, transmission power of HS-SCCH in the case of transmitting control information transmitted to each MS.
Japanese Laid-open Patent Publication No. 2006-197318 discloses that in a mobile radio communication system and radio communication apparatuses comprising at least a movable radio communication apparatus among a plurality of radio communication apparatuses, multiplexing channels comprising at least a traffic channel for transmitting packets, a control channel for transmitting control information for packet receiving, and a channel for transmitting information comprising transmission power control information, and performing radio communication between one radio communication apparatus such as a base station and the other radio communication apparatus such as a mobile station, the transmission power control information is transmitted by the channel for transmitting the information comprising the transmission power control information in a constant period longer than the length of a packet from the one radio communication apparatus to the other radio communication apparatus, and the transmission power control information is transmitted by the control channel when the packet is transmitted by the traffic channel.
International Publication Pamphlet No. WO 2004/114551 discloses that a transmission power controlling method wherein on a reception side, an error rate of received data is compared with a target error rate to control the target desired-wave to interference-wave ratio (SIR: Signal to Interference Ratio), and on a transmission side, the transmission power is controlled such that the measured SIR becomes coincident with the foregoing target SIR, the characteristics of correspondences between error rates and SIRs are measured and stored beforehand, and then used to obtain first and second SIRs corresponding to the target and measured error rates, respectively, and the difference between the first and second SIRs is used to raise/reduce the target SIR and thereby update the target SIR.
In the conventional communication of the W-CDMA system, as depicted in FIG. 12, a mobile terminal device (UE: User Equipment) 2 performs wireless communication as moving within a service area 6 in a base station (Node-B) 4. Therefore, the mobile terminal device 2 has a transmission and reception unit 8 that processes amplification and modulation/demodulation of a wireless signal transmitted and received to/from the base station 4, and a signal processing unit 10 that executes an encoding process, a decoding process, and so on. The base station 4 has a transmission and reception unit 12 that includes an amplifier and a modulation and demodulation part of a wireless signal transmitted and received to/from the mobile terminal device 2, and an interface 18 for connecting the signal processing unit 14 executing an encoding process, a decoding process, and so on to a network 16.
In the communication of the W-CDMA system, the mobile terminal device 2 transmits transmit power control (TPC) information at regular intervals by a link from the mobile terminal device 2 in order that a signal transmitted from the base station 4 to the mobile terminal device 2 is received at the mobile terminal device 2 with sufficient power and interference to other channels due to transmission with excessive power is minimized. The base station 4 executes transmit power control that increases and reduces transmit power of the transmission and reception unit 12 according to this TPC information. TPC information is information generated according to a reception level (quality) of a data signal and so on transmitted from the base station 4, and information for giving instructions of reducing transmit power if a reception level (quality) is high, and giving instructions of increasing transmit power if a reception level (quality) is low.
As enhancement of the wireless communication system of the W-CDMA system, F-DPCH (Fractional Dedicated Physical Channel) is standardized by 3GPP Release 6 (3rd Generation Partnership Project Release 6). This transmits TPC information by a dedicated channel, that is, includes TPC bit patterns (FIGS. 13 and 14) as TPC information.
As a frame structure including TPC information, FIG. 13 depicts frame structure of F-DPCH, FIG. 14 depicts frame structure of DPCH (Dedicated Physical Channel), FIG. 15 depicts frame structure of DPDCH (Dedicated Physical Data Channel) and uplink DPCCH (Dedicated Physical Control Channel) and FIGS. 16A-D depict transmit timings of DPCCH. DPDCH is user information and DPCCH is control information.
Since a user executing data reception by a common channel (traffic channel) such as HS-PDSCH transmits traffic data by HS-PDSCH, there is often a case that data to be transmitted by a dedicated channel (for example, DPCH different from an HS channel) is almost not present. Transmit power control needs connection of a dedicated channel (DPCH) simultaneously with HS-PDSCH along with transmission of TPC bit patterns. Therefore, one code is occupied although there is no need to transmit data. This results in resource shortage since there are a plurality of the same users. Thus, this resource shortage is dissolved by F-DPCH transmitting only TPC bit patterns to a user who does not have data to be transmitted by a desiccated channel.
As described above, in the W-CDMA system, TPC information is transmitted at regular intervals by an uplink from the mobile terminal device 2 in order that a signal transmitted from the base station 4 to the mobile terminal device 2 is received at the mobile terminal device 2 with sufficient power and interference to other channels due to transmission with excessive power is minimized. The base station 4 executes transmit power control that increases and reduces transmit power of the transmission and reception unit 12 according to this TPC information. This TPC information is generated according to a reception level (quality) of a data signal and so on transmitted from the base station 4, gives instructions of reducing transmit power if a reception level (quality) is high, and gives instructions of increasing transmit power if a reception level (quality) is low.
In a dedicated channel (DPCH), two types of high speed TPC control are executed for executing high accurate TPC control that are inner loop TPC control controlling SIR, which is measured, so as to be Target SIR that is required reception quality, and outer loop TPC control executing CRC calculation of DPCH for adapting to the environment of propagation of the number of transmission paths, a moving rate, etc. to correct Target SIR so as to be required BLER (Block Error Rate).
In TPC control of F-DPCH, SIR of TPC bit patterns and reception quality of F-DPCH are correlated since a CRC signal is not added to a data format of F-DPCH. Target SIR is set so as to be required F-DPCH reception quality, and the measured SIR of TPC bit patterns is controlled so as to be Target SIR.
For such control, a calculation process of ISCP (Interference Signal Code Power) and RSCP (Received Signal Code Power) of F-DPCH (FIG. 17) is needed. ISCP is interference wave reception power and RSCP is desired wave reception power. FIG. 17 depicts an image of a timing diagram of calculation processes of ISCP, RSCP and SIR in F-DPCH by the symbol.
For F-DPCH demodulation in the mobile terminal device 2 which is a mobile device, in order to demodulate a certain slot (slot #<n>), a channel estimation filter process from a reception wave of slot #<n−1> of CPICH (Common Pilot Channel) is needed. This process is for controlling uplink transmit power in accordance with a TPC command after 1024 [chips] from reception of F-DPCH. Also, measurement of reception quality of F-DPCH is needed for mapping control of a TPC bit pattern on upstream DPCCH transmitted after 1024 [chips]. For measuring reception quality of F-DPCH of slot #<n>, the channel estimation filtering process is executed from reception data of slot #<n−1> of CPICH to calculate ISCP and RSCP.
ISCP is interference power of a reception signal. This ISCP is calculated by a calculation process of a CPICH symbol received at the same time of reception time of a TPC symbol of F-DPCH of slot #<n> and a channel estimate calculated by the channel estimation filtering process from a reception wave of slot #<n−1> of CPICH. Therefore, an ISCP value deteriorates under the environments that the central time of channel estimation filtering is past time and that a channel estimation result varies for a short time in accordance with a fading environment, a multipath environment, etc. and the past channel estimate and the current channel estimate are different. Since ISCP is a physical quantity that does not effect on the fading environment and multipath environment of a mobile device inherently, the accuracy of ISCP deteriorates under the fading environment and multipath environment. Therefore, the SIR measurement accuracy of F-DPCH deteriorates, and the accuracy of conversion to Target TPC Command Error Rate deteriorates. A power control signal is transmitted from the mobile terminal device 2 to the base station 4. Since this power control signal is transmitted from the base station 4 to the mobile terminal device 2, an error rate of reception data of the mobile terminal device 2 can be detected in the mobile terminal device 2. This error rate is Target TPC Command Error Rate.
Conventionally, a conversion table depicting Target SIR corresponding to Target TPC Command Error Rate is used as depicted in FIG. 18 for obtaining Target SIR of F-DPCH corresponding to Target TPC Command Error Rate.
In such a conversion table (FIG. 18), necessary power control may not be executed because conversion to proper Target SIR may not be formed since a situation different from a normal environment such as a fading environment is not assumed.
If power control of F-DPCH may not be normally executed in the base station 4, power beyond necessity is transmitted. Excessive power transmission bears on wireless resources to lower throughput of a system. If F-DPCH is transmitted with power below the reception capability of the mobile terminal device 2, there occurs inconvenience that transmit power control may not be normally executed at the mobile terminal device 2 side.
Concerning such problems, there is no suggestion or disclosure thereof in Japanese Laid-open Patent Publication Nos. 2005-303836 and 2006-197318 and International Publication Pamphlet No. WO 2004/114551, and no disclosure about solving means thereof is presented.
According to an aspect of the embodiments, a method of transmit power control to allow a transmission side to execute transmit power control so that a signal to interference ratio (SIR) measured at a reception side is matched with a target SIR, the method includes digitizing a reception environment at the reception side, and setting the target SIR corresponding to an error rate of reception data for each of the reception environments; and changing the target SIR in accordance with the reception environment measured at the reception side or the reception environment selected at the reception side. According to such structure, power control can be executed with high accuracy in comply with a reception environment since the target SIR set by each reception environment is changed according to the change of the reception environment. Thus, the above object can be achieved.
According to another aspect of the embodiments, a device of transmit power control to allow a transmission side to execute transmit power control so that an SIR measured at a reception side is matched with a target SIR, the device includes a storage unit to be set for each reception environment at the reception side, and to store the target SIR corresponding to an error rate of reception data; and a control unit to update the SIR to the target SIR in accordance with the reception environment measured at the reception side or the reception environment selected at the reception side. Such structure can also achieve the above object.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
Other objects, features and advantages of the embodiments of the present invention become clearer by referring to the attached drawings and each of the embodiments.